1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a heat fixing apparatus to be mounted on an image forming apparatus, such as a printer or a copying machine, utilizing electrophotography technology or electrostatic recording technology.
In addition, the present invention relates to a rotatable heating member that is employed by the image heating apparatus.
As such an image heating apparatus, there are a fixing apparatus for fixing an unfixed image on a recording material, a glossiness enhancing apparatus for increasing a glossiness of an image by heating an image fixed on a recording material, and so on, for example.
2. Description of the Related Art
In the image forming apparatus, a fixing apparatus is used in order to heat and fix an unfixed toner image formed and carried on a recording material to the recording material. As such a fixing apparatus, the fixing apparatus of a contact heating type is widely used. In the contact heating type fixing apparatus, a rotatable heating member (hereinafter, referred to as fixing roller) heated at a predetermined temperature is brought into contact with the recording material in a nip portion. In this way, the unfixed toner image on the recording material is heated and fixed to the recording medium so as to form a fixed image.
The fixing roller may be heated by one of an inside heating type and an outside heating type (surface heating type). In the internal heating type, a heating unit (heat source: heater) is disposed in the fixing roller, and the fixing roller is heated from the inside thereof so as to heat the surface of the fixing roller at a predetermined fixing temperature. In the outside heating type, a heating unit is disposed outside the fixing roller, and the fixing roller is heated from the outside thereof so as to heat the surface of the fixing roller at a predetermined fixing temperature.
The outside heating type fixing apparatus heats the fixing roller from the outside thereof by a heating unit of a small heat capacity, such as a heat roller of a small diameter having a small ceramic heater or a halogen heater. According to the outside heating type fixing apparatus, it is possible to rapidly increase the surface temperature of the fixing roller, thereby reducing a warm-up time period, as compared with the internal heating type fixing apparatus.
Japanese Patent Application Laid-Open No. H06-075491 discusses a fixing roller (heat roller) for outside heating configured to reduce a time period required to increase temperature to a fixable temperature. That is, the fixing roller includes a core metal, a heat insulating layer, a heat transfer layer, and a surface layer (toner release layer). The heat insulating layer is made of silicon rubber to have elasticity at the outer periphery of the core metal. The heat transfer layer is made of a metallic material to have a thickness of 5 μm to 100 μm at the outer periphery of the heat insulating layer. The surface layer is made of fluorine resin to have a thickness of 5 μm to 30 μm at the outer periphery of the heat transfer layer.
The fixing roller has elasticity, such that a high-quality image with no fixing irregularity can be formed. In addition, the surface of the fixing roller is heated, and thus even if the elastic layer is provided below the surface layer of the fixing roller, it is possible to rapidly increase the surface temperature of the fixing roller.
Japanese Patent Application Laid-Open No. 2002-123117 and Japanese Patent Application Laid-Open No. 2004-317788 discuss a construction that a high thermal conductive silicon rubber layer is disposed between a heat insulating elastic layer and a release layer.
By increasing thermal conductivity near the surface layer of the fixing roller, heat from an external heating unit is easily transferred, and thus the surface temperature of the fixing roller is rapidly increased. Therefore, a warm-up time period can be reduced.
Japanese Patent Application Laid-Open No. 2004-101865 discusses an image forming apparatus including a fixing member and a pressure member pressed into contact with the fixing member, a recording material being heated in a pressure nip portion pinched therein. Each of the fixing member and the pressure member is provided with a heat insulating elastic layer, a high thermal conductive silicon rubber layer, and a high thermal conductive release layer. A heat roller serving as an external heating unit comes into contact with the fixing member and the pressure member. Then, heat is applied to both sides of the recording material, that is, a side on which an unfixed toner is carried (hereinafter, this is referred to as a foreside, and the other side is referred to as a backside), and the backside.
Japanese Patent Application Laid-Open No. 2004-101865 also defines thermal conductivity of a high thermal conductive solid rubber layer and a heat insulating elastic layer in order to reduce the warm-up time period. With this construction, the warm-up time period can be reduced not more than 60 seconds.
In the outside heating type, however, a heating unit is not provided in the image forming apparatus, or only the fixing roller is provided with a heating unit. Accordingly, if the fixing roller and the pressure member have small heat capacity in order to reduce the warm-up time period, the following problems may occur.
That is, when the fixing roller that is not provided with a heat source therein comes into contact with the recording material to transfer heat thereto, the surface temperature of the fixing roller is decreased. For this reason, if the surface of the fixing roller is not heated by the heating unit at a significantly high temperature, a sufficient heat amount may not be applied to the recording material.
The available surface temperature of the fixing roller surface is limited to be not more than a predetermined temperature in view of heat resistance of the fixing roller, a temperature detection element, and a safety element. For example, even if silicon rubber having high heat resistance is used for the elastic layer of the fixing roller, when the fixing roller is successively used at high temperature of not less than 230° C., rubber is deteriorated and may be destroyed. The fixing roller needs to secure a sufficient heat amount required for heating and fixing in the fixing nip portion at a temperature less than 230° C.
If a high thermal conductive layer is provided near the surface of the fixing roller, a temperature near the fixing roller can be effectively increased by the heating unit. In this case, however, if the surface temperature of the fixing roller required for heating and fixing is excessively high, the fixing roller may not be used safely and stably.
For this reason, if heat capacity near the surface of the fixing roller is not appropriate, in addition to internal heat insulation performance and high thermal conductivity near the surface, the fixing roller may not store a heat amount required for heating and fixing toner to the recording material.
The discussions on the known outside heating type fixing apparatus have not suggested the appropriate heat capacity near the surface of the fixing roller.
The discussions on the known outside heating type fixing apparatus relate to a heating apparatus in which the warm-up time period is approximately 60 seconds. At present, there is a need for an image heating apparatus in which power is not consumed during a standby state, and it takes a short time period from when a print signal is received until the recording material on which an unfixed toner image is formed is heated and fixed. This time period is usually not more than 20 seconds.
A low thermal conductive layer is provided in the fixing roller, and the fixing roller is heated from the surface thereof. However, the expected warm-up time period is so long that heat reaches the inside of the fixing roller, not only the surface thereof. In this case, it is not taken into consideration that heating and fixing are performed with a heat amount stored near only the surface of the fixing roller within a shot warm-up time period of not more than 20 seconds.
The known outside heating type heat fixing apparatus has not been embodied as an apparatus that does not consume power during the standby state, sufficiently reduces the time period from when the print signal is received until the recording material on which the unfixed toner image is formed is heated and fixed, and achieves sufficient fixing performance for the recording material.
The time period from when the print signal is received until the recording material on which the unfixed toner image is formed is heat fixed is hereinafter referred to as a first print-out time period.